1. Field of the Invention
The present invention generally relates to digital video apparatuses, and more particularly, to a method for reducing channel changes times in a digital video apparatus.
2. Background Information
In a digital television broadcast system, allowing viewers to change channels rapidly is an important feature. There are at least two major sources that contribute to channel change delay. The first source is from the group of picture (GOP) structure used by many video compression standards, such as MPEG-2 and H.264. In a GOP, pictures are encoded using either intra or inter coding. An intra coded (I) picture (a.k.a., frame) uses information within itself for compression, and therefore can be decoded alone. Inter-coded pictures (i.e., B or P pictures), however, must be decoded after the related intra coded pictures (I picture) are decoded. Since I pictures typically require 3 to 10 times more bits than a B or P picture, they are encoded much less frequently in the bit stream in order to reduce the overall bit rate. In a digital television broadcast system, I frames may only appear once every 1 to 2 seconds. When a digital video apparatus tunes to a program, it must wait until the first I frame is received before any picture can be decoded or displayed. This can cause a significant delay.
The second source for channel change delay is from processing components, such as buffering, forward error correction (FEC) and error concealment in the digital video apparatus (e.g., set-top box (STB)). These components require significant buffering and processing, and therefore cause delay in channel change. For an example, traffic shaping is desirable for transporting variable bit rate (VBR) streams, but it would require buffering at the digital video apparatus, which causes delay. FEC requires buffering of video packets for packet loss recovery. The digital video apparatus needs to provide buffering to hold at least one block of video packets and their associated FEC packets before decoding. 20-column and 5-row FEC protection for a 3 Mbps constant bit rate (CBR) stream may, for example, introduce a delay as high as 350 milliseconds. Error concealment, as another example, requires the detection of loss of frames, which is a delayed event. Moreover, advanced error concealment algorithms, such as optical flow based processes, offer better concealment results, but are much more computationally intensive than the traditional frame repeat method. In a modern digital video apparatus, concealing 4 consecutive frames could create a delay as long as 200 milliseconds. In order to display video without significant jitter, all these delays have to be budgeted into the digital video apparatus even though they may not always be required.
Accordingly, in view of the potentially lengthy delays associated with channel changes, there is a need for a method for reducing channel changes times in a digital video apparatus. The present invention described herein addresses this problem, and is particularly useful for dealing with channel change delays associated with signal processing within a digital video apparatus.